Mu receptor distributions and pharmacologic properties place them among the receptors most identified with the analgesic and addicting properties of opiate drugs. In previous FYs, cDNAs encoding rodent mu opiate receptors have identified by our group. Availability of these cDNAs have allowed us to define the structure of the human mu opiate receptor cDNA and gene, identify human kappa opiate receptor cDNAs, identify closely-related "orphan" receptor cDNAs, and compare pharmacological features of mu opiate receptors between the human and rodent receptors. This work defines, for the first time, the structure of this primary site of opiate drug euphoria, addiction and analgesia in humans.